I discovered an interesting feature of Amazon.com yesterday. I was searching for FC contributing writer Martin Kihn's new book, the extremely smart and funny "multinational phenomenon" (Marty's words) "House of Lies: How Management Consultants Steal Your Watch and Then Tell You the Time."

There it was--and even though the official pub date isn't until next week, its Amazon "sales rank" was already #980. Pretty good. I happened to check back literally a minute later, though, and the sales rank had slipped to #1083. (Uh-oh.) Another two minutes, and "House of Lies" had rocketed to #889. (Yay!) And two hours after that, it was at #661!

As of two minutes ago, "House of Lies" had dropped back to #1275.

Amazon's help section explains only that its sales rank "calculation is based on Amazon.com sales and is updated each hour to reflect recent and historical sales of every item sold on Amazon.com. We hope you find the Amazon.com Sales Rank interesting!"

We do! But clearly, it's updated more often than once an hour. Does it reflect the sales average of the most recent five minutes?